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@danielwangksu
Forked from parse/shell.c
Created August 4, 2017 22:55
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Simple shell in C
/* Compile with: g++ -Wall –Werror -o shell shell.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
/* The array below will hold the arguments: args[0] is the command. */
static char* args[512];
pid_t pid;
int command_pipe[2];
#define READ 0
#define WRITE 1
/*
* Handle commands separatly
* input: return value from previous command (useful for pipe file descriptor)
* first: 1 if first command in pipe-sequence (no input from previous pipe)
* last: 1 if last command in pipe-sequence (no input from previous pipe)
*
* EXAMPLE: If you type "ls | grep shell | wc" in your shell:
* fd1 = command(0, 1, 0), with args[0] = "ls"
* fd2 = command(fd1, 0, 0), with args[0] = "grep" and args[1] = "shell"
* fd3 = command(fd2, 0, 1), with args[0] = "wc"
*
* So if 'command' returns a file descriptor, the next 'command' has this
* descriptor as its 'input'.
*/
static int command(int input, int first, int last)
{
int pipettes[2];
/* Invoke pipe */
pipe( pipettes );
pid = fork();
/*
SCHEME:
STDIN --> O --> O --> O --> STDOUT
*/
if (pid == 0) {
if (first == 1 && last == 0 && input == 0) {
// First command
dup2( pipettes[WRITE], STDOUT_FILENO );
} else if (first == 0 && last == 0 && input != 0) {
// Middle command
dup2(input, STDIN_FILENO);
dup2(pipettes[WRITE], STDOUT_FILENO);
} else {
// Last command
dup2( input, STDIN_FILENO );
}
if (execvp( args[0], args) == -1)
_exit(EXIT_FAILURE); // If child fails
}
if (input != 0)
close(input);
// Nothing more needs to be written
close(pipettes[WRITE]);
// If it's the last command, nothing more needs to be read
if (last == 1)
close(pipettes[READ]);
return pipettes[READ];
}
/* Final cleanup, 'wait' for processes to terminate.
* n : Number of times 'command' was invoked.
*/
static void cleanup(int n)
{
int i;
for (i = 0; i < n; ++i)
wait(NULL);
}
static int run(char* cmd, int input, int first, int last);
static char line[1024];
static int n = 0; /* number of calls to 'command' */
int main()
{
printf("SIMPLE SHELL: Type 'exit' or send EOF to exit.\n");
while (1) {
/* Print the command prompt */
printf("$> ");
fflush(NULL);
/* Read a command line */
if (!fgets(line, 1024, stdin))
return 0;
int input = 0;
int first = 1;
char* cmd = line;
char* next = strchr(cmd, '|'); /* Find first '|' */
while (next != NULL) {
/* 'next' points to '|' */
*next = '\0';
input = run(cmd, input, first, 0);
cmd = next + 1;
next = strchr(cmd, '|'); /* Find next '|' */
first = 0;
}
input = run(cmd, input, first, 1);
cleanup(n);
n = 0;
}
return 0;
}
static void split(char* cmd);
static int run(char* cmd, int input, int first, int last)
{
split(cmd);
if (args[0] != NULL) {
if (strcmp(args[0], "exit") == 0)
exit(0);
n += 1;
return command(input, first, last);
}
return 0;
}
static char* skipwhite(char* s)
{
while (isspace(*s)) ++s;
return s;
}
static void split(char* cmd)
{
cmd = skipwhite(cmd);
char* next = strchr(cmd, ' ');
int i = 0;
while(next != NULL) {
next[0] = '\0';
args[i] = cmd;
++i;
cmd = skipwhite(next + 1);
next = strchr(cmd, ' ');
}
if (cmd[0] != '\0') {
args[i] = cmd;
next = strchr(cmd, '\n');
next[0] = '\0';
++i;
}
args[i] = NULL;
}
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